Compact is more condensed, I think.

--astype.de--

Actually, FF Dax Condensed is about the same width as FF Dax Compact. The main difference is the latter has...

"relatively small ascenders and descenders and the uppercase letters have the same height as the lowercase ascenders. This makes the typeface optically larger -- useful for headlines and posters where words must shout." - Hans Reichel

Typically the terms compact and compressed refer to a width narrower than condensed. This width is almost always used only for large display sizes. Condensed can be used (in small doses) for text.
From what Stephen says, I infer that FF is using the compact description to indicate that it is a headline font.

ChrisL

Ahh ok - That makes sense.
Thank you.